1. Field of the Invention
This invention relates to a machine for manufacturing a fibrous pulp web, in particular a tissue or hygienic paper web, with a main press comprising a clamping unit and a drying cylinder.
2. Discussion of Background Information
DE-A-42 24 730 describes several embodiments of a paper machine for processing a tissue or hygienic paper web. One of these embodiments consists of three press locations, with two pre-pressing units having top rolls and lower shoe press rolls having flexible roll sleeves, as well as a main press with a contact press roll and a drying cylinder. The pre-pressing rolls are located before the main press, in the direction of web travel. The paper web is produced on a roll formed between an upper belt and a drainage sieve. The lower belts, designed as permeable felt belts, are guided together with the paper web and the upper belt through both pre-pressing units.
In another known embodiment, an upper belt receives a paper web and subsequently guides it through a pre-pressing unit between an upper roll, and a lower roll with a lower felt, to a main press between a contact press roll and a drying cylinder. In still another known embodiment, the contact press roll is a shoe press roll.
Except for their use of shoe presses, these embodiments more or less correspond to the former, conventional two-felt belt tissue arrangements. These, however, are no longer used in new hygienic paper machines.
In conventional two-felt machines, the upper felt is very dense and the lower felt is very absorbent. To achieve maximum removal of water by the lower felt, the lower roll is designed as a suction roll.
A problem with such two-felt machines, which has led to their replacement by one-felt machines, is that at increasing run speeds, preliminary drying does not occur quickly enough, even with the use of a suction press. As a result, there is inadequate discharge of water, and the web is crushed in the pre-pressing unit.
Nevertheless, the use of a shoe press for the pre-pressing unit is fundamentally advantageous. The specific embodiments shown in DE-A42 24 730, however, have the disadvantage that, among other things, the respective shoe press rolls are arranged in the lower position, where a very wet paper web, for example having a dry content of approximately 6% to 8%, may come from the sheet-formation screen.
Due to the limited depths of the blind holes or grooves on the upper surface of the press sleeve, which result from manufacturing design limitations, the press sleeve cannot absorb during operation all of the accumulated water. This may be the reason that in most of the specific embodiments described in DE-A42 24 730, the main press is arranged with two pre-pressing units, and in one case, a suction press roll is provided as well. Furthermore, in most of the known embodiments, an upper belt is employed which is water impermeable or waterproof.
Especially when compared to a one-felt machine, known embodiments of two-felt machines take up a relatively large amount of space, particularly where two pre-pressing units and a main press are provided, which don't remove any water on the crape cylinder.